Crude edible oils and fats as obtained from oilseeds or oil-bearing fruits consist mainly of various triacylglycerols (triglycerides) but they also contain non-triglyceride components. Some of these are considered to be desirable such as the tocopherols that act as anti-oxidants and have vitamin E activity, but others such as free fatty acids, phospholipids and malodorous compounds have to be removed. For this purpose several refining processes have been developed. Degumming processes aim at removing phospholipids from the crude oil; bleaching processes aim at removing residual phospholipids and colouring matter by treating the oil with an adsorbent, and high-temperature vacuum stripping processes aim at removing crude oil constituents that are less volatile than the triglycerides.
In this context, a distinction must be made between commodity oils such as but not limited to soya bean oil, palm oil, rapeseed oil, and sunflower seed oil, on the one hand, and speciality oils and fats such as but not limited cocoa butter, olive oil, various cold pressed oils, and gourmet oils in general on the other. Whereas commodity oils should be bland and almost colourless, speciality oils and gourmet oils in general should reflect their agricultural origin and may therefore have a characteristic colour and/or flavour that should be retained during refining.
Another distinction to be made concerns the manner in which the free fatty acids are removed from the crude oil. In the chemical refining process, the free fatty acids (FFA) are removed by making them react with sodium hydroxide when treating the crude oil with caustic soda (lye). This converts the FFA into soaps that can be separated from the oil by using a centrifugal separator. A disadvantage of this process is that the soap phase entrains a fair amount of neutral oil, which constitutes a refining yield loss. Another disadvantage is that the acidulation of the soap phase leads to an aqueous effluent with a high sulphate and phosphate content and a high chemical oxygen demand. Accordingly, the physical refining process, in which the FFA are removed as such by volatilization during a high-temperature vacuum stripping process is gradually replacing the chemical refining process.
Because the neutral oil loss during chemical refining is almost proportional to the FFA-content of the crude oil, the advantages of the physical refining process over the chemical refining process are most pronounced for high-FFA oils such as palm oil. Crude palm oil has a low phospholipid content, which means that these contaminants can be effectively removed in the so-called dry degumming process as disclosed in U.S. Pat. No. 4,089,880. In this process, the crude oil is treated with a degumming acid such as phosphoric acid. This acid decomposes the so-called non-hydratable phosphatides (NHP) present in crude oil, so that they can subsequently be removed by adsorption onto bleaching earth. This bleaching earth also removes colouring compounds. Accordingly, applying the dry degumming process and the physical refining process provides a simple purification process for low-phosphatide oils like palm oil, lauric oils like palm kernel oil and coconut oil and animal fats like lard and tallow. It has the advantages of consisting of only two steps and low neutral oil entrainment and hence giving a high yield.
Since those skilled in the art firmly believe that physical refining can only be applied to oils with a low (<3 ppm P) phosphatide content, the advent of effective degumming processes for seed oils, such as the acid refining processes disclosed in U.S. Pat. No. 4,698,185 and U.S. Pat. No. 5,239,096, meant that seed oils also became amenable to physical refining. This started with medium-FFA oils such as sunflower seed oil and rapeseed oil, but its use gradually spread to low-FFA oils such as soya bean oil, because the scale of refinery operations had evolved to such high capacity plants that even a relatively small process improvement led to large absolute savings. These improvements not only relate to oil yield but in the case of vacuum stripping also to energy and steam requirements and by-product valorization.
In respect of energy savings much progress has been made in continuous deodorisers by the introduction of countercurrent heat exchangers. As disclosed in U.S. Pat. No. 6,001,220, stripping steam requirements have been reduced by the introduction of packed columns that allow countercurrent steam stripping. Using less stripping steam also means that less motive steam is required in the steam ejectors or, if a mechanical vacuum pump is used to maintain the vacuum, less electrical energy is required.
Packed columns have a disadvantage in that they present a resistance to the vapour flow which results in a pressure drop over the column. Thus, if the oil to be deodorised or physically refined is first allowed to trickle down a packed column before being further steam stripped in the deodorisation trays below said column, the pressure above these trays will be substantially higher than the pressure above said column. Since in steam stripping the steam requirement is proportional to the absolute pressure, the steam savings resulting from the use of a column are to a large extent lost because of the increased steam requirements of the trays.
US 2005-066823A1 provided a solution to this problem by disclosing a vacuum stripping process for the physical refining or deodorization of a liquid material, the process being performed in an apparatus comprising at least a first stripping vessel being supplied with said liquid material to be vacuum stripped and also being supplied with a gaseous stripping medium by a single pump that obtains this gaseous stripping medium from at least one further downstream second stripping vessel, the at least one second stripping vessel being equipped with two or more trays, over which a partially stripped material coming from the first stripping vessel is directed, wherein said partially stripped material on each of these trays is sparged with fresh gaseous stripping medium and said gaseous stripping medium is collected from above each tray of the at least one second stripping vessel by means of the pump. An additional pump is thereby provided that draws vapours from above the trays, compresses these vapours, and feeds the compressed vapours below the packed column resulting in the pressure above the trays being reduced so that they require less stripping steam to achieve the same degree of volatile removal.
Improvements have also been made with respect to the valorisation of by-products. In deodorisation and physical refining it is customary to pass the vapours leaving the oil through a vapour scrubber and condense the organics. These organics are a mixture of different compounds comprising free fatty acids, tocopherols, non-esterified sterols, malodorous compounds and various compounds that are specific for the agricultural origin of the oil being processed. As only to be expected, the FFA content of this mixture depends strongly on the FFA content of the oil that is going to be stripped under vacuum. When this oil has a high FFA content of say 5%, the FFA content of the condensate may be as high as 90% or even higher but when a neutral oil is deodorised the amount of condensate is much smaller and its FFA content is lower as well.
Since the value of this condensate in the market depends on its purity, processes have been developed to isolate high-value components from this condensate, such as for instance tocotrienol compounds as disclosed in US 2002/0142083A. U.S. Pat. No. 6,750,359B1 discloses a process for isolating components from a vaporized distillate, comprising: (a) introducing a vaporized distillate comprising sterols, tocopherols, and fatty acids into a first condensing zone of a condensing unit having at least two condensing zones and operating at a pressure of less than about 10 mm Hg, the first condensing zone operating at a temperature of from about 330 to about 450° F. (about 165.6 to 232.2° C.); (b) condensing a first fraction of the vaporized distillate in the first condensing zone to produce a first condensate enriched in sterols and tocopherols, leaving a remaining fraction of vaporized distillate; (c) introducing the remaining fraction of vaporized distillate into a second condensing zone of the condensing unit, the second condensing zone operating at a temperature of from about 100 to about 170° F. (about 37.8 to about 76.7° C.); and (d) condensing a second fraction of the remaining fraction of vaporized distillate in the second condensing zone to produce a second condensate enriched in fatty acids, leaving a waste vapor. Such fractional condensation using two scrubbers operating at different temperatures as disclosed in U.S. Pat. No. 6,750,359B1 can also lead to purer condensates with increased market value.
Instead of fractional condensation, a fractional evaporation has also been suggested. In 1988, A. Athanassiadis in a paper entitled “The deacidification of vegetable oils by distillation during deodorization” published in Fat. Sci. Technol., volume 90, pages 522-526, suggested a separate deacidifier preceding a standard tray deodoriser. Three types of deacidifier are shown in this paper: a tube and shell type in which the oil can be heated when flowing down the tube walls, a packed column that requires the oil to be pre-heated, and a column with superimposed stages. Stripping medium is supplied to all three types and the vacuum in the deacidifiers is the same as in the deodorisers.
U.S. Pat. No. 4,599,143 discloses a process for removing lower boiling components from at least one liquid member selected from the group of high-boiling, organic, edible oils; high-boiling, organic, edible fats; high-boiling, organic, edible esters; or mixtures thereof; employing continuous countercurrent falling film stripping steam distillation in an externally imposed termperature field, which comprises heating said liquid member to 220° to 280° C. at a working pressure between 2 and 10 mbar, causing said liquid member at 220° to 280° C. and said working pressure to flow down as a thin film having a film thickness of less than 1.0 mm at the wall of substantially vertically arranged surfaces forming trickle passages maintaining at least a part of said passages at a higher temperature than the downflowing liquid, and passing a vapor of a low-molecular weight liquid countercurrently to said liquid through said trickle passages; which comprises providing at least two countercurrent falling film stripping steam distillation zones operating in series and being in vapor and liquid communication with each other and providing an unrestricted vapor flow connection and wherein the hydraulic equivalent diameters of the trickle passages in the final distillation zone downstream in the direction of the downflowing liquid are smaller than the hydraulic equivalent diameters of the trickle passages of the initial zone(s) located upstream of said final distillation zone; and supplying stripping steam exclusively to the bottom of said final distillation zone. In a preferred embodiment of the process disclosed in U.S. Pat. No. 4,599,143, the liquid to be treated has a relatively high proportion of free fatty acids and other low-boiling components, and the process further comprises carrying out a flash treatment immediately before entry into the initial distillation zone, and wherein said flash treatment is conducted at substantially the same working pressure as in the initial zone; and separating and condensing the vapors released upon the flashing. The stripping steam supplied below the second falling film column passes also through the tubes of the first falling film column but does not pass through the flash vessel. Instead, it passes immediately to a scrubber.
US 2005-066823A1, in a preferred embodiment, also discloses that the liquid material to be vacuum stripped be exposed to vacuum in a flashing vessel before being vacuum stripped in the first stripping vessel. The vessel can be outside or inside the deodoriser shell and no stripping medium is supplied to this vessel. The pressure inside the vessel is the same as above the packed column used for the countercurrent vacuum stripping treatment since the vapours leaving the flash vessel and those leaving the packed column enter a communal vacuum system through the same scrubber.
US 2004-0210070A1 discloses a vegetable oil processing system, comprising: a deodorizer having a first deodorizer segment and a second deodorizer segment, the first and second deodorizer segments being maintained at an elevated temperature; a first vapor outlet in fluid communication with the first deodorizer segment and positioned to receive a first vapor stream comprising a majority of first volatiles from the first deodorizer segment; a second vapor outlet in fluid communication with the second deodorizer segment and positioned to receive a second vapor stream which comprises second volatiles from the second deodorizer segment; a first recovery system in fluid communication with the first vapor outlet and adapted to cool the first vapor stream to preferentially condense a first recovered fluid and direct a remaining first byproduct stream outwardly through a first byproduct outlet; a second recovery system in fluid communication with the second vapor outlet and adapted to cool the second vapor stream to preferentially condense a second recovered fluid and direct a remaining second byproduct stream outwardly through a second byproduct outlet; a common byproduct condenser in fluid communication with the first and second byproduct outlets, the byproduct condenser being maintained at a pressure of no greater than 10 mm Hg and being adapted to condense a substantial fraction of the first and second volatiles remaining in the first and second byproduct streams; and a common vacuum system in fluid communication with and adapted to maintain pressures in the first and second deodorizer segments, the first and second recovery systems, and the common byproduct condenser at a pressure no greater than 10 mm Hg. This system also has a common vacuum system but instead of a single scrubber, it comprises several different scrubbers. One of these scrubbers is connected to the central chimney that is surrounded by deodorisation trays. Another separate scrubber is connected to the first tray that is used to heat the oil to deodorisation temperature. Since these scrubbers are separate, their condensates can also be kept separate. Because the different scrubbers are connected to the common vacuum system, they operate at the same pressure.